How good is your magneto – and does it produce a spark at the plugs under compression?
Have you got suppressors fitted (which should not be so with a magneto). Also, I trust you have proper copper cored H.T. leads – not modern carbon trace leads.
Other than that, Roger could be on the right track saying the compression suffers when hot – but if you have tried “hill starts” and the like, the engine speed at cranking would be high enough to overcome any dimensional alterations which might otherwise affect starting.
Are you sure your bikes not been sabatarged to stop you going out so you can spend more time on producing “Yowl”?